Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Did you know that people (scientists included) thought that mice came from wheat and old shirts? Learn about theory of spontaneous generation and the centuries-long debate over its validity.

What Is Spontaneous Generation?

Have you ever noticed that after it rains you see a lot more worms? We now know that worms tend to stay underground because their skin needs to be moist so they can breathe. When it rains, the ground is covered in water, so they are free to move about. However, people used to believe that worms, rather than hatching from the eggs of other worms, were created when dirt and water made mud.

This is the idea of spontaneous generation, an obsolete theory that states that living organisms can originate from inanimate objects. Other common examples of spontaneous generation were that dust creates fleas, maggots arise from rotting meat, and bread or wheat left in a dark corner produces mice.

Although the idea that you can create scorpions by placing basil between two bricks and leaving them in the sun seems ridiculous to us now, the theory of spontaneous generation was hotly debated for hundreds of years. During this time, many experiments were conducted to both prove and disprove the theory. We will discuss the research of Francesco Redi, John Needham, Lazzaro Spallanzani and Louis Pasteur.

Francesco Redi

In 1668, Francesco Redi published the first set of experiments challenging spontaneous generation. In these experiments, he demonstrated that maggots arise from the eggs of flies rather than directly from rotting meat. He did so by taking several jars and placing raw meat inside of them. Half the jars he left open to the environment and the other half he covered with gauze.

Several days later, the open jars had maggots in the meat, while the jars that were covered, making it impossible for flies to lay their eggs inside, had none. Additionally, he noted that there were maggots on the outside of the gauze on the covered jars. He was thus able to show that the rotting meat did not generate the maggots.

John Needham

John Needham, however, was not convinced. He argued that Redi did not fully disprove the theory of spontaneous generation. Needham maintained that while spontaneous generation did not occur in larger organisms, for smaller organisms, it could still take place under the right circumstances. In 1745, he set out to prove this by boiling broth in a flask, leaving it open to cool and then sealing it. In several days time, the broth was teeming with bacteria and mold. He claimed that this was proof of the existence of spontaneous generation for microorganisms.

Lazzaro Spallanzani

Lazzaro Spallanzani reviewed Needham's work and discovered several glaring errors in his experimental design: he did not boil the broth long enough to kill the spores of the microbes, and by leaving the flask open to the air, new microorganisms were able to enter. In 1765, Spallanzani repeated the experiment with some important changes. He boiled two separate flasks for an extended period, sealed one immediately and left the second open to the air. The open flask developed microorganisms, while the sealed flask did not. Would this disprove the theory of spontaneous generation for good?

Unfortunately, this did not happen. Scientists argued that air was a requirement for spontaneous generation, and by sealing the flask, Spallanzani deprived the environment of the necessary air. So, really, his experiment proved nothing. The debate would thus continue for another 100 years.

Louis Pasteur

In 1864, nearly 200 years after Redi published his experiments, Louis Pasteur would finally settle the dispute once and for all. Pasteur conducted the broth experiment for a third time, but this time, he created flasks with long, S-shaped necks. These flasks could be left open so that air could reach the boiled broth, but they were twisty enough that microbes could not access the inside of the flask. Pasteur was able to leave his flask for several weeks, and still no microorganisms appeared.

Finally, he broke off the neck, allowing the microbes to fall from the air into the broth and multiply. Thus, Pasteur proved beyond a doubt that microorganisms did not spontaneously form in the broth, but rather they were the products of parent microbes that fell into the mixture through gravity. The theory of spontaneous generation was put to rest for good.

Lesson Summary

Spontaneous generation is the outmoded theory that living organisms, rather than coming from the reproduction of their species, arise from nonliving objects. This theory was widely debated in the scientific community for hundreds of years. Francesco Redi, John Needham, Lazzaro Spallanzani and Louis Pasteur were some of the notable scientists who debated this issue from 1668 to 1864.

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